Multi-layer printed circuit boards are widely used in modern electronics technology to interconnect various components in a compact and efficient manner. These components include active Integrated Circuits (ICs) or passive discrete components (resistors, capacitors, and the like). As electronic circuits become more complex and use a larger number of components, printed circuit boards must typically accommodate more electronic components in the same surface area. As a result, electronic components have become densely located and modern printed circuit boards use multiple layers to provide various supply and return voltages such as 5 V, 3.3 V, and Ground (GND). In addition, these supply and return voltage planes are used as impedance control planes (also referred to as “reference planes”) for controlling the characteristic impedance of interconnecting signal routing. Modern printed circuit boards typically use multiple signal routing layers (“signal layers”) adjacent to the reference planes for routing signals between components. As larger number of electronic components are used and densely located on printed circuit boards, these electronic components generate an increasing level of noise associated with power distribution among the electronic components. Various techniques (generally termed “decoupling”) have been developed so far to somewhat control the noise generated on multi-layer printed circuit boards.
Capacitors have been widely used in electronic circuits to control noise in power distribution. For example, decoupling capacitors have been widely used in printed circuit boards for controlling noise from active devices on printed circuit boards. Conventional decoupling capacitors are selected and placed based upon techniques such as local decoupling, bulk decoupling, and embedded (or “buried”) capacitance.
Local decoupling requires that capacitors be placed between the supply voltage (Vcc) and the return voltage (typically GND) connections of an active electronic component. As such, local decoupling capacitors provide a local source of instantaneous current for active devices (also referred to as “parts”). By supplying charge for instantaneous current demand close to the active devices, it is possible to limit the amount of transient voltage disturbance generated in the power distribution close to these actives devices. Local decoupling capacitors are selected based upon analysis of the instantaneous current requirement and the acceptable transient noise between the power supply and return voltage connections of each part and are placed as close as possible to that part's power supply connection pins on each part (hence, “local decoupling capacitors”). Local decoupling capacitors typically range in value from 0.001 μF to 22 μF and are placed as close as possible to the power and ground pins of the individual parts.
Bulk decoupling serves the same purpose as local decoupling, but uses larger value capacitors that are typically placed further away from the parts. In other words, bulk decoupling uses larger capacitance values associated with a group of parts rather than individual parts and the bulk decoupling capacitors are placed further away from each individual part. Bulk decoupling also limits the transient noise generated on the Vcc/GND power distribution. Bulk capacitors are typically in the range of 47 μF to several 10,000 μF.
Embedded capacitance employs closely spaced power/ground plane pairs to build a low inductance capacitor for decoupling power/ground transients from active devices. Embedded capacitance is generally more effective at higher frequencies than standard capacitor construction, and due to the relatively low capacitance value (typically less than 0.01 μF) its purpose is typically to provide more effective high frequency local decoupling for active devices. U.S. Pat. No. 5,162,977 to Paurus et al., entitled “Printed Circuit Board Having an Integrated Decoupling Capacitive Element” describes the technique of building buried capacitance in detail. Embedded capacitance is more effective at higher frequencies than local decoupling or bulk decoupling, but its purpose is limited to controlling high frequency transient noise on the power/ground distribution generated by active devices.
As explained above, all conventional decoupling capacitors such as local decoupling, bulk decoupling, and buried capacitance, are effective only for controlling transient noise on the power/ground distribution generated by active devices. In other words, conventional decoupling capacitors are selected and placed based solely upon consideration of the power/ground noise characteristics and/or the power/ground pin locations of the active devices. The conventional decoupling capacitors are not selected based upon analysis of the interconnecting signal information or the interconnecting signal route information. Therefore, the conventional decoupling capacitor placement does not control the noise generated by the specific signal path (also referred to as “signal traces”) of interconnecting routes between active devices.
In conventional systems, interconnecting signal traces are routed as traces over a plane (called “microstrip” or “stripline” transmission lines). In the typical situation on a multi-layer PCB, an interconnecting signal trace is routed on a signal layer and the immediately adjacent voltage or ground plane is used as the impedance control/signal return plane. A circuit model analysis of current flow in the transmission line structure shows that, at high frequencies, the majority of signal return current flows as the mirror image in the impedance control plane, where the total impedance of the return path in the plane is lowest. Using a field model for the microstrip or stripline transmission line structure, it is understood that the electromagnetic field density is greatest in the region of dielectric captured between the signal trace and the plane, and much lower outside of this region. Thus, the energy transfer (current flow in the circuit model) between devices occurs in the region of the PCB between the signal trace and its impedance control plane.
On modern printed circuit boards with multiple supply voltages and ground planes, the path of the return current becomes problematic when the signal trace transitions routing layers and uses different planes for impedance control. This is a particular problem if the impedance control planes used are not the same (i.e. coupled to a different potential), because in such situation the path to the nearest decoupling capacitors will determine the locations for allowing the return current to transition between reference planes. For optimum noise control and signal integrity, the signal return current should have a location for transitioning that is close to the signal trace layer transition (called the signal “via”). FIGS. 1–3 described below show how the signal return current on the conventional signal path can create serious problems for signal integrity on multi-layer printed circuit boards.
FIG. 1 is a diagram illustrating a first conventional multi-layer printed circuit board 100 including four layers, a Vcc1 supply voltage plane 102, a Vcc2 supply voltage plane 104, a ground (GND) plane 106 with conventional local decoupling capacitors 128, 130, 132, and signal routing layer in which the signal current 134 is routed. Two active devices, device 108 (driver part), device 110 (receiver part), and another active device 112 (unrelated to devices 108 and 110) are placed on the printed circuit board 100. The devices 108, 110, 112 may be any type of device such as an active integrated circuit (IC) chip or a discrete component. Device 108 has at least three pins 114, 116, 118, device 110 has at least three pins 120, 122, 124, and device 112 has at least two pins 125, 126. Devices 108 and 110 are powered by the Vcc1 plane 102, while device 112 is powered by the Vcc2 plane 104. Device 108 is connected to the Vcc1 plane 102 via pin 114, to the GND plane 106 via pin 116, and to device 110 via pin 118. Device 110 is connected to the Vcc1 plane 102 via pin 120, to the GND plane 106 via pin 122, and to device 108 via pin 124. Device 112 is connected to the Vcc2 plane 104 via pin 126 and to the GND plane 106 via pin 125.
Capacitors 128, 130, 132 are conventional local decoupling capacitors connected between the voltage source planes and ground planes to which devices 108, 110, 112 are connected. Specifically, capacitor 128 is a local decoupling capacitor connected between pin 114 and pin 116. Capacitor 130 is a local decoupling capacitor connected between pin 120 and pin 122. Capacitor 132 is a local decoupling capacitor connected between pin 125 and pin 126.
A signal current (solid line) 134 flows on the signal path in a routing layer between pin 118 of device 108 to pin 124 of device 110 with the current propagating in the routing layer routed adjacent to part of the GND plane 106. Here, the GND plane 106 is called the “reference plane” or “impedance control plane” for the routing layer in which the signal current 134 flows. As can be seen in FIG. 1, the current signal between pins 118 and 124 is routed adjacent a reference plane 106 to which both the devices 108 and 110 are coupled. As such, the return current (dotted line) 136 required according to Kirchoff's first law can flow back from device 110 to device 108 via pins 122, 114, on part of the GND plane 106 (reference plane), with the connection to the local decoupling capacitor 128 providing the return current path between pin 122 of device 110 and pin 114 of device 108. Therefore, as shown in FIG. 1, the local decoupling capacitor 128 alone is capable of providing the return current path for the signal path between pin 118 and pin 124, when the signal current 134 is routed against a reference plane to which the driver device and the receiver device are coupled. There is no need for additional bypass capacitors, because the loop area of the signal path is already minimized by the connecting the conventional decoupling capacitors 128, 130. It is obvious from FIG. 1 that device 112 or its local decoupling capacitor 132 plays no role in providing a return current path between devices 108, 110 for controlling noise in the signal path therebetween.
FIG. 2 is a diagram illustrating a second conventional multi-layer printed circuit board 200 including four layers, a Vcc1 supply voltage plane 202, a Vcc2 supply voltage plane 204, a GND plane 206 with conventional local decoupling capacitors 230, 232, 234, and a routing layer in which a signal current 236 is routed. Two active devices, device 208 (driver part), device 210 (receiver part), and another active device 212 (unrelated to devices 208 and 210) are placed on the printed circuit board 200. The devices 208, 210, 212 may be any type of device such as an active integrated circuit (IC) chip or a discrete component. Device 208 has at least three pins 214, 216, 218, device 210 has at least three pins 220, 222, 224, and device 212 has at least two pins 226, 228. Devices 208 and 210 are powered by the Vcc1 plane 202, while device 212 is powered by the Vcc2 plane 204. Device 208 is connected to the Vcc1 plane 202 via pin 214, to the GND plane 206 via pin 216, and to device 210 via pin 218. Device 210 is connected to the Vcc1 plane 202 via pin 222, to the GND plane 206 via pin 224, and to device 208 via pin 220. Device 212 is connected to the Vcc2 plane 204 via pin 226 and to the GND plane 206 via pin 228.
Capacitors 230, 232, 234 are conventional local decoupling capacitors connected between the voltage source planes and ground planes to which devices 208, 210, 212 are connected. Specifically, capacitor 230 is a local decoupling capacitor connected between pin 214 and pin 216. Capacitor 232 is a local decoupling capacitor connected between pin 222 and pin 224. Capacitor 234 is a local decoupling capacitor connected between pin 226 and pin 228.
In the printed circuit board 200 of FIG. 2, a signal trace (solid line) 236 represents current that flows on the signal path in the routing layer between pin 218 of device 208 to pin 220 of device 2 with the current routed in the routing layer adjacent to part of the Vcc2 plane 204 for impedance control. Vcc2 plane 204 is not GND and does not power the devices 208, 210. This type of signal routing is a common practice that permits for improved routing efficiency in multi-layer printed circuit boards. Here, the Vcc2 plane 204 operates as an alternate reference plane (or impedance control plane) for the signal current (solid line) 236.
As can be seen in FIG. 2, the current signal 236 between pins 218 and 220 is routed in a routing layer routed adjacent to a reference plane 204 that does not power the devices 208 and 210. The devices 208 and 210 are not coupled to the Vcc2 reference plane 204. As such, the return current (dotted line) required according to Kirchoff's law cannot flow back directly from device 210 to device 208 via pins 224, 214, because the return current does not have a local path near the devices 208 and 210 for transitioning between reference planes. Rather, the return current has to find a separate local decoupling capacitor 234 far away from the devices 208, 210 and coupled between the Vcc2 reference voltage plane 204 and the GND plane 206 for transitioning between reference planes. This local decoupling capacitor 234 for device 212 is unrelated to the signal current 236 and is located far away from devices 208, 210, but nevertheless has to provide the return current path between pins 224 and 214. Thus, the return current has to flow along a long path 238, 240, 242, 244, 246, 248, 250, 252, and 254, resulting in a large and uncontrolled return current path that leads to (i) a large loop area created by the signal current and its return current, and (ii) a large difference in path length between the signal current and its return current path. Large loop area and large difference in path length can result in increased crosstalk between active circuits due to increased inductive coupling (increased “loop area”), and in increased signal transmission line reflection due to impedance discontinuities at the signal transition locations. The increased length of the return path relative to the signal path also results in unbalance. All of these factors deteriorate signal integrity. Signal transmission line reflection due to impedance discontinuities, inductive crosstalk to other signals due to a larger loop area formed by the signal trace and its return current, and imbalance due to a difference in path length of the signal trace and its return current are all generally referred to as “reduced signal integrity” herein. Therefore, the conventional local decoupling capacitors 230, 232 alone are not capable of controlling the noise on the signal path between pin 218 and pin 224, because the signal cur rent is routed adjacent to an impedance control plane not powering or serving as the circuit reference (GND) between the driver and receiver devices 208, 210.
FIG. 3 is a diagram illustrating a third conventional multi-layer printed circuit board 300 including six layers, a Vcc1 supply voltage plane 302, a first GND plane 304, a Vcc2 supply voltage plane 306, and a second GND plane 308, with conventional local decoupling capacitors 326, 328, and two routing layers in which the signal currents 330 and 361 flow. Two active devices, device 310 (driver part) and device 312 (receiver part), are placed on the printed circuit board 300. The devices 310, 312 may be any type of device such as an active integrated circuit (IC) chip or a discrete component. Device 310 has at least three pins 314, 316, 318, and device 312 has at least three pins 320, 322, 324. Both devices 310 and 312 are powered by the Vcc1 plane 302. Device 310 is connected to the Vcc1 plane 302 via pin 314, to the GND plane 304 via pin 316, and to device 312 via pin 318. Device 312 is connected to the Vcc1 plane 302 via pin 322, to the GND plane 304 via pin 324, and to device 310 via pin 320.
Capacitors 326, 328 are conventional local decoupling capacitors connected between the voltage source planes and ground planes to which devices 310, 312 are connected. Specifically, capacitor 326 is a local decoupling capacitor connected between pin 314 and pin 316. Capacitor 328 is a local decoupling capacitor connected between pin 322 and pin 324. These capacitors are connected to planes 302 and 304.
In the printed circuit board 300 of FIG. 3, a signal current (solid line) 330 flows on the signal path between pin 318 of device 310 to pin 320 of device 312 with the interconnecting signal flowing in a routing layer routed adjacent to parts of both the Vcc2 plane 306 and the second GND plane 308 for impedance control. The Vcc2 plane 306 does not power the devices 310, 312. Thus, the signal current 330 is routed in the routing layer adjacent to reference planes (impedance control planes) not powering the devices 310, 312, and is also routed in routing layers adjacent to two reference planes (impedance control planes) 306, 308. The signal current 330 transitions between reference planes or impedance control planes by way of “routing vias” 360, 362. This use of routing vias affords improved routing efficiency in multi-layer printed circuit boards as the signal trace can be routed on multiple signal layers.
As can be seen in FIG. 3, the current signal between pins 318 and 320 is routed in a routing layer adjacent to two reference planes 306, 308. The devices 310 and 312 are not coupled to the Vcc2 reference plane 306. Thus, there are no capacitors connected to the Vcc2 plane 306. As such, the return current (dotted line) required according to Kirchoff's first law cannot flow back directly from device 312 to device 310 via pins 324, 314, because the return current does not have a local path near the devices 310 and 312 for transitioning between reference planes. Specifically, the return current may have paths from pin 324 to point 336 along path 334, from point 338 to point 342 along path 340, from point 344 to point 348 along path 346, from point 350 to point 354 along path 352, and from point 356 to pin 316 along path 358. However, there is no return current path between point 336 and point 338, between point 342 and point 344, between 348 and point 350, and between point 354 and point 356. Rather, as in the printed circuit board of FIG. 2, the return current has to find an arbitrary (uncontrolled) decoupling capacitor(s) (not shown) potentially far away from the devices 310, 312 and coupled between the Vcc2 plane 306 and the GND planes 304, 308 in order to transition between these reference planes. These local decoupling capacitor(s) (not shown) are unrelated to the signal current 330 and may be located far away from devices 310, 312, but nevertheless have to provide the missing return current paths between pins 324 and 314. Thus, the return current (dotted line) follows a long and uncontrolled return current path that leads to (i) a large loop area created by the signal current and its return current, and (ii) a large difference in path length between the signal current and its return current path. This long return path can deteriorate signal integrity. The conventional local decoupling capacitors 326, 328 alone are not capable of controlling noise and signal integrity on the signal path between pin 318 and pin 320, because there are routing vias 360, 362 in the signal current path between the driver and receiver devices.
Therefore, there is a need for a method for controlling noise and improving signal integrity in multi-layer printed circuit boards based upon analysis of the interconnecting signal information or the interconnecting signal routing information. Furthermore, there is also a need for a method for selecting and placing bypass capacitors for controlling noise and improving signal integrity in multi-layer printed circuit boards based upon analysis of the interconnecting signal information or the interconnecting signal route information.